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Weekly Seminars for April 2010 Print E-mail

Thursday 1st April 2010 - 4:00 P.M.

Università "La Sapienza" Roma - Aula Majorana (Physics Dept., Old Building - Ground Floor)

Speaker:  Dr. Marco Pizzi (Physics Dept., University of Rome “Sapienza” and Physik Dept., Freie Universitat Berlin)

Title: On the incompleteness of the Unruh modes in Minkowski spacetime 

Abstract: A review is given about the role of the boundary conditions in the Unruh "effect". Following the works by Narozhny, Fedotov, Karnakov, Mur, and Belinski, it is showed that the Unruh quantization has a meaning only on the "double wedge", i.e. on the right and left sector of Minkowski spacetime, with a zero boundary condition on the light cone.Such a condition is incompatible with the quantization of a free field in Minkowski spacetime. Dropping out this boundary condition the Unruh quantization can still have a meaning only if the light cone it cutted out. Again, in this case the Unruh modes constitute an incomplete basis for the whole Minkowski spacetime. Connections with the Hawking effect are also discussed.


Wednesday 7th April 2010 - 3:00 P.M.

Università "La Sapienza" Roma - Aula 5 (Physics Dept., New Building - 2nd Floor)

Speaker:  Dr. Stefan Gillesen (Max-Planck-Institut für extraterrestrische Physik)

Title: The Massive Black Hole in the Galactic Center 

Abstract: Since the discovery of the non-thermal, compact radio source Sgr A* it has been suspected that the center of the Milky Way harbors a massive black hole. A breakthrough in measuring the associated mass became possible with the advance of infrared astronomy. Within a decade the observations have progressed from velocity dispersion arguments to tracking individual stars as test particles for the gravitational potential. Technically this was possible due to high- angular resolution techniques, in particular adaptive optics, at a very large telescope. The observations revealed a star that moves apparently on a Keplerian orbit with a period of 15 years, allowing to measure the mass with unprecedented accuracy. Today, the 4 million solar masses black hole in the Galactic Center is the best case for the existence of an astrophysical black hole in general. For more than twenty stars orbits have been determined and further progress can be expected - both from the ever growing time base as well as from future techniques such as near-infrared interferometry. Equally fascinating are the flares occurring a few times per day, during which Sgr A* shines up in the infrared and X-ray domain. The emission is due to a population of transiently heated electrons in the innermost zone of the accretion flow onto Sgr A*. With the advent of near-infrared interferometry in the future, such events could be used as tracers for the space-time around the massive black hole.


Thursday 8th April 2010 - 4:00 P.M.

Università "La Sapienza" Roma - Aula Majorana (Physics Dept., Old Building - Ground Floor)

Speaker:  Dr. Cristine Nunes Ferreira (Instituto Federal de Educação Ciência e Tecnologia Fluminense - Campos dos Goytacazes, Brazil)

Title: Some aspects of the D dimensional Brans-Dicke theory: Topological defect configuration and electroweak model 

Abstract: We consider a scalar field interaction with a topological defect configuration. The origin of the scalar field is given by a compactification mechanism in the context of a D dimensional Brans-Dicke theory. We analyse the behavior of a charged cosmic string given by the Maxwell-Chern-Simons on the 3-brane. The Cosmic Microwave Background Radiation constraint is used to analyse a possibility of optical activity effect in connection with the Brans-Dicke parameter. We show that the dilatons produced by a cosmic string can decay into gauge bosons with masses given by the compactification modes. The Brans-Dicke parameter imposes stringent constraints on the mass of the dilaton and help us to understand the energy scales. In this scenario the lifetime of the dilaton which decays into light gauge bosons as well as the dependence of this phenomenon with the Brans-Dicke parameter are estimated. We also discuss the effect of the Brans-Dicke parameter on a recent Higgless electroweak model.


Thursday 15th April 2010 - 4:00 P.M.

Università "La Sapienza" Roma - Aula Majorana (Physics Dept., Old Building - Ground Floor)

Speaker:  Prof. Marco Tavani (IASF-Roma, INAF)

Title: Highlights from the AGILE mission 

Abstract: In this talk I will report on the latest results obtained with the AGILE satellite.


Thursday 22nd April 2010 - 4:00 P.M.

Università "La Sapienza" Roma - Aula Majorana (Physics Dept., Old Building - Ground Floor)

Speaker:  Prof.ssa Rita Bernabei (Dipartimento di Fisica, Università Tor Vergata - INFN)

Title: Recenti risultati di DAMA-LIBRA 

Abstract: Recenti progressi nell'esperimento saranno illustrati con particolare attenzione a possibili sorgenti di Materia Oscura.


Tuesday 27th April 2010 - 4:00 P.M.

Università "La Sapienza" Roma - Aula Careri (Physics Dept., Old Building - 1st Floor)

Speaker:  Prof. Lev Titarchuk (Physics Dept., University of Ferrara; ICRANET and Goddard Space Flight Center - USA)

Title: X-ray spectral index correlations vs mass accretion rate in neutron star and black hole X-ray binaries in their different spectral states. Theory vs observations 

Abstract: I present details of the first principle theory of X-ray spectral formation in neutron star (NS) and black hole (BH) binaries. I show this theory predict the spectral index correlation vs mass accretion rate as in the case of NS as well in the BH case. In both cases this index correlation is mostly correlated to the ratio of energy release in the disk vs that in transition (boundary) layer in a BH case. In BHs the spectral index should increase and then saturate with mass accretion rate because the index as an inverse of Comptonization parameter Y and Y-parameter saturates with the high mass accretion in the converging flow onto BH. Comparison of this model prediction with X-ray observations shows that in BH case the index, indeed, correlates and then saturates with mass accretion rate. Moreover this index-mass accretion rate correlation allows us to estimate BH masses and distance to the source. While in NS sources the observations shows that the index stays the same independently of spectral state of the source which can be possible if the energy release in the disk is always much smaller of that at NS TL (boundary layer).


 
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